Qt is a comprehensive, object-oriented development framework that enables development of high-performance, cross-platform rich-client and server-side applications. When you implement a program with Qt, you can run it on the X Window System (Unix/X11), Apple Mac OS X, and Microsoft Windows NT/9x/2000/XP by simply compiling the source code for the platform you want. Qt is the basis for the KDE desktop environment, and is also used in numerous commercial applications such as Google Earth, Skype for Linux, and Adobe Photoshop Elements.

GTK, which stands for the Gimp ToolKit, is a library for creating graphical user interfaces. It is designed to be small and efficient, but still flexible enough to allow the programmer freedom in the interfaces created. GTK provides some unique features over standard widget libraries.

WebGUI is a content management framework built to
allow average business users to build and maintain
complex Web sites. It is modular, pluggable, and
platform independent. It was designed to allow the
people who create the content to manage it online,
rather than content management taking up the time
of busy IT staff. WebGUI comes with a full host of
features including shopping cart, subscriptions,
forums, photo galleries, FAQs, link lists, blogs,
SQL reports, a Web services interface, and a very
configurable user privilege and profiling system.

FLTK (pronounced "fulltick") is a cross-platform C++ GUI toolkit for UNIX/Linux (X11), Microsoft Windows, and Mac OS X. It provides modern GUI functionality without the bloat, and supports 3D graphics via OpenGL and its built-in GLUT emulation. FLTK is designed to be small and modular enough to be statically linked, and also works fine as a shared library. It also includes an excellent UI builder called FLUID that can be used to create applications in minutes.

Wt is a C++ library for developing Web
applications with an API that is widget-centric
and inspired by existing C++ graphical user
interface APIs. To the developer, it offers
abstraction of Web-specific implementation details, increasing the accessibility and portability. Under the hood, the library uses the latest techniques (HTML5, Ajax, WebSockets) to handle user events and update the Web page.

FOX is a C++-based toolkit for GUI development. It includes a rich set of widgets and has powerful yet simple layout managers, MDI widgets, and mega-widgets. FOX incorporates support for XDND for drag and drop, X clipboard and X Selection, watching other I/O channels and sockets, timers and idle processing, object serialization and deserialization, a registry to save persistent settings, and 3D widgets using Mesa or OpenGL. FOX works on Linux, IRIX, Solaris, HP/UX, AIX, Tru64 Unix, Windows 9x,NT,2K (VC++, GNUWIN32, Borland, VisualAge C++), FreeBSD, and Sequent.

qooxdoo is a universal JavaScript framework that enables you to create applications for a wide range of platforms. With its object-oriented programming model you can build rich, interactive applications (RIAs), native-like apps for mobile devices, traditional Web applications, or even applications that run outside the browser. Its integrated tool chain allows you to develop and deploy applications of any scale, while taking advantage of a comprehensive feature set and a state-of-the-art GUI toolkit.